Frequency drift compensation



SePt- 23, 1958 o. K. NlLssEN 2,853,614

FREQUENCY DRIFT COMPENSATION Filed Jan. 31, 1957 t (c) wenn doff/rr INVENToR.

L K. /V/LssE/v l FfEQz/EA/cr g I r BY 7744 Y, M /j/ww A Tram/EY United States Patent O aaa FREQUENCY DRIFT COMPENSATION Ole K. Nilssen, Inkster, Mich., lassignor to Radio Corporation of America, a corporation of Delaware Application January 31, 1957, Serial No. 637,49() 14 ClalllS. (Cl. Z50- 36) This application relates broadly to frequency drift compensation circuits, and particularly to an oscillator frequency drift compensation` arrangement employing elements possessing a large negative temperature coefficient of resistance.

Frequency stability in apparatus operating at very high frequencies (V. H. F.) and ultra high frequencies (U. H. F.) is critical. Many oscillators which employ electron discharge devicesand operate in these frequency ranges have a tendency to drift from their initially-set operating frequency, a rather appreciable period of time elapsing before a stable condition is reached. Investigation has shown that oscillator drift during the warm-up period is largely due to heat flow, both inside as Well as outside the oscillator electron discharge device or tube. The oscillator drift may be considered to undergo two transistory phases until stability is achieved. The initial phase, which accounts for mostl of the drift in frequency during a relatively short period of time, is due to physical changes Within the tube produced by the heat. The second phase, which accounts for a lesser part of the frequency drift spread over a much larger time period, is due to the changes taking place in the associated oscillatory tank circuit elements. The heat ow within the tube raises the temperature of the elements therein, changing` their configuration and geometry which. leads to changes of grid-to-plate, grid-to-cathode, and other interelectrode capacitances. It has been found that these tube capacitances exhibit a positive temperature coefficient. The generated heat increases the interelectro-de capacitances, thereby decreasing the oscillation frequency. It has also been found thatv the heat conducted from the tube by the oscillatory tank circuit inductance and changes in the ambient temperature are key factors in the long-term or second phase of frequency drift.

Metals commonly used in oscillator tank circuits possess a positive coefficient of expansion and thus an increase in temperature results in an increase in length, hence an increase in inductance. The change of inductance has been found to be directly proportional to thev average temperature rise. Additionally, in V. H. F. and U'. H. oscillators the circuit inductance usually consists of a metallic conductor of various'di'mensions and shapes, possessing a high thermal conductivity. The conductor isterminated-byV a capacitor serving ak dual purpose of partial frequency determinationand'compensation. In such circuits the circuitV inductance becomes the medium of heat conductivity between the tube and the tank circuit capacitance. The physical characteristics of this conduction medium are important factors affecting the oscillator frequency characteristics if a temperature-sensitive circuit capacitance is used. A complete analysis' of the -frequency characteristics of oscillators under conditions of' heat flow'is' presented in an article entitled Frequency characteristics of" local oscillators by W. Y. Pan, beginning orr page 379 of the September 1955 issue of the' RCA Review, volume XVI, No. 3,

-tli published by RCA Laboratories, Radio Corporation of America.

The instant invention proposes an `arrangement for causing an oscillator to exhibit a positive frequency drift characteristic of such an amount and nature as to compensate for or neutralize both the short-time and longtime negative frequency drift characteristics resulting from the heat ow factors previously discussed. The significant element of the arrangement proposed is a thermistor, a thermally sensitive resistor possessing a large negative temperature coefficient of resistance.

It therefore is an object of this invention to provide a novel means for stabilizing the frequency of an oscillator.

Another object of this invention is to provide a novel means for automatically compensating for oscillator frequency drift.

Still another object of this invention is to maintain the operating frequency of an oscillator constant over wide variations in component and ambient operating temperatures. Yet another object of this invention is to maintain the operating frequency of an electron discharge device or vacuum tube circuit at its initial value over an extended period of time.

In one aspect, the invention comprises the novel arrangement of a thermistor in the circuit suppl-ying voltage to the plate electrode of the oscillator tube so mounted as to compensate for long and short time frequency drift. As the initial operating temperatures Within the oscillator tube cause rapid changes in the interelectrode capacity, hence realization of a negative frequency drift characteristic, a correspondingly rapid heat generation from a primary heat source raises the temperature of the thermistor, the resistance of Whichdecreases with the temperature rise. A decrease of resistance of the thermisto-r, which is serially connected in the plate voltage supply circuit, lessens the voltage drop across the thermistor, raising the potential, at the oscillator tube plate electrode. Inasmuch as an oscillator exhibits a change in frequency depending upon the plate voltage, the increase in plate potential offsets the negative frequency drift characteristic caused by heat generation within the tube. The ther-mister is also arranged in the circuit in a novel manner so as to be responsive to heat radiated from a second source which follows the long-time temperature characteristic of the circuit. long-time increase in temperature, which lowers the oscillator frequency as a result of changes in'oscillatory tank circuit elements, will likewise decrease the resistance of the thermistor and raise the plate electrode potential. With selection of a proper thermistor this raises the oscillator frequency a sufficient amount to offset the heatproduced negative frequency drift. It is proposed that the primary heat source comprise a resistance inserted in the lanode electrode supply 4lead which will experience a rise in temperature as a result of current flow therethrough. The secondary heat source may be the inductance of the tank circuit, which will follow the long-time temperature characteristic of the circuit. ln the manner thusdecribed it is possible to obtain with one compensating element complete frequency stabilization of an oscillator, frequency drift compensation being introduced for both long-time and short-time variations in oscillator frequency due to the inuence of-heat.

These and other objects, aspects, features land advantages of the invention will be apparent to those skilled in the art from the following more detailed description taken in conjunction with the appended drawing, wherein:

Figure l is a combination electrical schematic diagram The additionalof an oscillator employing a thermistor element so mounted as to effect frequency drift compensation over a wide range of operating temperatures;

Figure 2 shows a series of curves illustrating the longtime, short-time, and overall frequency drift characteristics of 'an uncompensated oscillator due to temperature changes with respect to time;

Figure 3 shows a curve indicating the frequency deviation characteristic 4of an oscillator with respect to changes in anode voltage;

Figure 4 graphically illustrates the negative temperature coefficient of resistance of typical thermistors;

Reference is made to Figure l, illustrating an ultra high frequency (U. H. F.) triode oscillator employing the frequency drift compensation arrangement of this invention. Y

Such an oscillator may be utilized as the local oscillator in frequency modulation (F. M.) and television (TV) tuners operating in the V. H. F.U. H. F. portion of the frequency spectrum. An electron discharge `device such as a vacuum tube is used, having a plate electrode 18, a control grid 16, and a cathode 12. Since this triode is of the indirectly-heated type, the heater element 14 is also shown. The cathode 12 is connected to a point of reference potential, or ground, thru a radio frequency choke coil 2t) the function of which is to prevent the circulation of radio frequency currents in this cathode circuit. The heater voltage supply (not shown), lone terminal of which is grounded, is connected to the heater voltage terminal 27 and the heater circuit is completed thru a ground connection. Radio frequency choke coils 22, 24 are inserted in the heater circuit to allow the passage of direct current from the heater supply but prevent the circulation of radio frequency energy. A feed-through capacitor 25 serves to by-pass to ground any radio frequency energy appearing `in the heater circuit. The grid electrode 16 is connected to ground thru a direct current or D. C. grid-return resistance 26 serving as a grid-biasing means. An oscillatory tank circuit comprises two parallel inductors 32, 28 forming `a modified transmission line type tank. The plate 18 is -connected to one end of conductor 32, while the grid 16 is connected to the corresponding end of conductor 28. The other ends of inductors 32, 28 vare connected to opposite sides of a variable capacitor 36. The potential on the plate or anodey electrode 18 is applied from the positive terminal 42 (B+) fof an external unidirectional source, not shown. The terminal 42 connects to a radio frequency (R. F.) choke 40, which prevents the circulation of radio-frequency currents from the oscillatory tank circuit to the high-voltage power supply. The choke 40 connects to a heat-radiating resistance element 38which in turn is attached to a metallic mass 36 possessing a high thermal conductivity. The mass 36 may be of any shape or form convenient for mounting upon the thermistor element 34. It may be metallic, as described in this embodiment, a ceramic or any other material capable of acting as a good heat conductor; i. e., possessing a high thermal conductivity. Alternatively, a heat-conducting mass may be physically combined with a thermistor in a manner wherein the casing of the thermistor is composed of a thermallyconductive material such as a ceramic.

The mass 36 is mounted upon the thermistor element 34, which in turn mounts upon the plate-connected parallel rod inductor 32 near the plate end. The thermistor 34 is a thermally sensitive resistor possessing a large negative temperature coeicient of resistance. It may be commercially obtained in many shapes and sizes. The resistance element is usually a semi-conductor material employing a metallic oxide such as manganese, nickel, and cobalt. A complete discussion on such devices will be found in an article entitled Properties and uses of thermistors-thermally sensitive resistors, 1. A. Becker et al., beginning on page 7ll of the November 1946 issue of Electrical Engineering Magazine. In the embodiment shown, the first resistance element 38 is electrically connected to the metallic mass 36 which connects thru the thermistor 34 to the plate electrode 18. The principal function of the mass 36 is to transmit heat to the thermistor 34; hence it -is not essential that the resistor 38 be directly connected to the mass 36 and this resistor 38 may be directly connected to thethermist'or 34, provided the mass 36 receives heat generated by the plate resistor 38.

The oscillator circuit shown is the equivalent of a Colpitts oscillator, theinter-electrode and stray -capacitances associated with the electron discharge device 10 forming the capacitance divider network necessary in such as circuit. In a Colpitts circuit an exciting voltage of the appropriate phase is obtained by connecting the grid and plate electrodes to opposite ends of the tank circuit with respect to the cathode electrode, with the ratio of exciting voltage to alternating plate-cathode voltage being determined by the relative reactances on the two sides of the cathode connection; that is, by the values of the interelectrode and stray capacitances forming the capacitance'v divider. A discussion of such oscillators may be found beginning at page 480, in Radio Engineers Handbook, by Terman, first edition, McGraw-Hill Book Co., Inc. In operation, the oscillator of Figure l will exhibit a negative frequency drift characteristic 4due to heat flow both within and without the electron discharge devi-ce 10.

In Figure 2, which illustrates the frequency drift versus i time characteristics for an uncompensated oscillator, the

curves are representative of `those shown in Frequency characteristics of local oscillators, September 1955, RCA Review, cit. supra, which presents a complete analysis of the frequency characteristics of oscillators under conditions of heat ow. A further study of this problem is found in Minimizing locall oscillator drift, by W. Y. Pan and D. I. Carlson, beginning at page 21, RCA Engineer, December 1956-] anuary 1957, vol. 2, No. 4. It will be noted that there is an initial or short-time frequency drift of considerable magnitude, designated asl curve (a) of Figure 2, relative to the initial frequency, designated fo on the graph.` This portion corresponds to the heat-actuated changes within the tube which change the internal configuration and geometry, hence capacitance. The long-time or vsecondary drift, which is much smaller in magnitude 4and extends overa much longer period of time, is principally due to changes in external circuitry caused by both heat conducted from the tube and the temperature of the environment. This is designated as curve (b) of Figure 2. Curve (c) indicates the overall drift and is the sum of curves (a) and (b).

On the other hand it has been found that the operating frequency of such an oscillator, within limits, is dependent upon the value of the plate voltage for the vacuum tube. As indicated by'Figure 3, which illustrates this characteristic, the frequency will increase as the plate voltage ris increased. If the plate voltage can therefore be made'to change in a manner opposite to the changes brought about by the heat-flow, complete frequency drift v compensation may be achieved.

r change its resistance initially in a manner following the short-time temperature increase and subsequently in accordance with the long-time temperature change.

In the arrangement shown in Figure 1, the thermistor 34 s serially connected in the circuit supplying po-v tential to the plate 18 of the electron discharge device orv vacuum tube 10. A current ow from vthe supply terminal 42 passes `thru the first resistance 38, thence thru the metallic mass 36 and thermistor 34 to the plate electrode 18. Currentflow thru the rst resistor 38, which maintains substantially constant resistance -over a wide range of temperatures, causes the radiation of heat en-lv ergy to the metallic mass 36. The mass 36 heats rativev direction" toV compensate for the negative frequencyV drift caused by heat-produced' electrode expansion' within the tube;

The lower portion of thethermistor 34 being mounted on the plate-connected tank inductor 32, the long-time temperature of the circuit as 'reflected by the tank inductance 32 will: further influence the resistance of the thermistor 34' in a manner comparable to the changes brought about by'the' long-'timel temperature rise of the associated portions of the oscillator circuit; The' plate voltage' will be raised an amount necessary to compensate for the long-time negative temperature coeficent' of frequency of an otherwise'uncompensated oscillator, and is adjusted by selecting the proper thermistor characteristic. n

The instant invention isk not limited to' the physical arrangement shown but`Will" be equallyoperative where an element possessingal negative' temperature coeflicient of resistance locatedE in' a plate'voltage supply circuit is subjected to a heat sourcewhich isv representative of the circuit short-time temperature characteristic, as Well as another heat source representative of thelong-time temperature characteristic.

While the subject invention is described in connection with an U. H. F. triode oscillator', it willl betunderstood that it ist equally operable with otherv types of oscillators possessing a` negative temperature coeiicient of frequency. Particular utility isfound in" radar applications utilizing electron discharge devices such as the voltagetunable magnetron and theY reflex-klystron which are quite frequency sensitive with respect to voltage variations. Additionally, the'inventi'onl is not confined to oscillatory circuits, but isJ applicable toy any tuned circuit associatedv with the operating circuit' off althermioni electron discharge device.

It is to be understoodv thatl the formV of the invention, herewith shown and described, isto be' takenv as-a preferred example of the same', and that various changes in the shape, s'ize-and-arrangementfof the parts may lbe resorted to, Without departing from' the spirit of thefinvention and of the scope of the subjoined claims.v

What is claimed' is:

1. A temperature compensatedf electron/discharge device circuit comprising, an electron discharge device including a plateelectrode', anoscillatoryl tank circuitconnected to said plate` electrode', a source" of unidirectional current, a resistanceA element exhibiting a negative temperaturel coeicient of resistancev serially connected be'- tween said source and said plate electrode, a heat source actuated by said source of unidirectional current and in thermal-coupling relation to said resistance element, said resistance element being responsive to heat generated by said heat source.

2. A temperature compensated electron discharge device circuit comprising, an electron discharge device including a plate electrode, an oscillatory tank circuit connected to said plate electrode, a source of unidirectional current, a resistance element exhibiting a negative temperature coeicient of resistance, a heat source in thermalcoupling relation to said resistance element, means connecting said resistance element and said heat source in series between said source of unidirectional current and said plate, said resistance element being responsive to heat generated by said heat source.

3. A temperature compensated electron discharge device circuit comprising, an electron discharge device including a plate electrode, an oscillatory tank circuit connected to said plate electrode, a source of unidirectional The lessened resistance in the4 current, a resistance element exhibiting a negative tcmperature coefiicient ofl 'resistanceY serially connected between said source and-said plate electrode, aheatv source actuated by said" source ofunidirectional'y current and* in` thermal-coupling relationA to saidfresist'ance' elementi said resistancev element beingy responsive to heat` gener'- ated by said heat source, and a mass of material having' highj thermal conductivity abutting saidY resisance ele;-A ment and adapted tobel heated' by saidheatsource.

4i A temperature compensated? electron discharge' device circuit! comprising, an electron' dischargeV device inf -cluding a' plate electrode, an` oscillatory tank circuiti ature co'efcient of frequency, `a source of unidirectional' current, av first` resistance element exhibiting a' negative ktemperature coefficient of resistance, a heat producingL second resistance element, means serially' connecting said' first and second resistance elements to said source of unidirectional current and a point Iintermediate the, ends of s'aid inductance of said oscillatory tank circuit', said' first-Y resistance element being mounted upon and abutting said inductance of said: tank circuit, whereby said firstresistance elernentisarranged to'respond to heat transl mitted by said second resistance andk to'ambient tem'- peratureva'riations transmitted by said inductance.

5. In an oscillation generator, a frequency compensating system comprising, a unidirectionalk source of cur'- rent, a lirstresistance element exhibiting substantially constant resistance over a wide range of temperatures, a second resistanceeleme'ntexhibiting a negative temperature coefficient of resistance, said iirst resistance element electrically connecting said second resistance element to said current source, a massY of high thermal conductivity mounted@inheat-transmittingrelationship upon said second resistance, said mass arranged toy receive heat radiated from said iirst resistance, an oscillatory tank circuit including an element at ambient temperature, said tank circuit being electrically connected to said second resistance element so thatsaid second' resistance element is mounted in heat-receiving relationship upon' said tank' circuit element.

6. In an oscillation generator, ay circuit comprising, arrv electron discharge device including an input electrode andl anv output electrode, an oscillatory tank circuit including an' induct'an'ce element, said tank circuitbeing connectedlbetween'said input and output electrodes, a rst resi-s'tancefelementi exhibit-ing a negative temperature coefficient offre'sistancersaidiirst resistance element being in electrical' connection with s'a'id: oscillatory tank circuit and-thru saidftankcirc'uitwith` said output electrode, said iirstresistancev elementlh'aving'one side mounted'in heatreceiving; relationship to-said tank induct'anceclement, a' rnas's" exhibiting af highl thermal conductivity characteristic mounted in heat-transmitting relationship to another side of said rst resistance element, a second resistance element in electrical connection with said first resistance element, said second resistance element being mounted in heat-transmitting relationship to said mass, a source of unidirectional current, said second resistance electrically connecting said current source to said oscillatory tank circuit thru said iirst resistance element.

7. A circuit as claimed in claim 6 wherein said mass is electrically connected between said rst resistance element and said second resistance element.

8. In a frequency stabilized oscillator including an electron discharge device, an oscillatory tank circuit connected thereto and determining the frequency thereof, said electron discharge device and said tank circuit together exhibiting a negative temperature coeiiicient of frequency, and a source of unidirectional current: a positive temperature coeicient of frequency control circuit comprising, a first resistance element, a second resistance element, said rst resistance element exhibiting a negaresistance over afwide variation in temperature, said irstv and second resistance elements electrically connecting said source of unidirectional current to said electron discharge Vdevice and said tank circuit, a mass exhibiting a high thermal conductivity characteristic mounted in heatreceiving relationship to said second resistance element and in heat-transmitting relationship to said first resistance element, said rst resistance element being mounted inheat-receiving relationship to said tank circuit.

9. A temperature-actuated electron discharge -device voltage control circuit comprising, a current-actuated primary heat source, a secondary heat source at ambient temperature, aY resistance element exhibiting a negative temperature coei'licient of resistance, a mass exhibiting a high thermal conductivity, said mass being arranged in` a heat-receiving relation with said primary heat source and ay heat-transmitting relation with said resistance element, said resistance element being arranged in a heatreceiving relation with both said primary and said secondary heat sources, a second source of unidirectional potential, meansfor connecting said primary heat source and said resistance element in series between said second source and said electron discharge device.

l0. Inrcombination, an electron discharge device hav-V ing a'plurality of electrodes, a unidirectional power source, a connection from a terminal of said source to one` of said electrodes, a thermistor, a rapidly-heating resistance element proximately arrangedv thereto, said resistance element and said thermistor in series in said connection, and means connecting said electrodes to a resonant circuit whereby oscillations are set up in said circuit, the frequency of said oscillations tending to drift as the temperatures of said electron ldischarge deviceelectrodes and of the ambient surrounding said device and circuit change, the frequency` of said oscillations changing in response to changes in the voltage applied to said one electrode, said thermistor being exposed to heat radiation from said proximately arranged resistance element and to the ambient temperature, thereby to vary the voltage applied to said one electrode in response to variations in said temperatures.

l1. In combination, an electron discharge device having an anode electrode and other electrodes; a unidirectional power source, a connection from a terminal of said source to said anode electrode, a rst resistance element exhibiting a zero temperature coeicient of resistance, a

second resistance element exhibiting a negative temperature coelicient of resistance, said rst resistance element being rapidly heating and being arranged in heat-trans` mitting relationship to said second resistance element, said first and second resistance elements in series in said connection, and means connecting said electrodes to a Cil resonant circuit whereby oscillations are set up inv said circuit, the frequency of said oscillations tending to drift as the temperatures of the electron discharge device electrodes and the ambient surrounding said device and circuit changes and the frequency of said oscillations changing in response to changes in the voltage applied to said anode electrode, said second resistance element being arranged to receive Vheat'radiation from both said rst resistance element and the ambient, thereby to vary-the voltage applied to said anode electrode in response to variations in said temperature.

12. In an oscillation generator employing a paralleltuned oscillatory circuit and wherein the operating voltage controls the generated frequency, a thermistor in series with the operating voltage, a heat-supplying element in circuit with said operating voltage for supplying heat to said thermistor, and means providing a second heat ilow path for vsaid thermistor by which heat may be removed from said oscillatorycircuit and transferred to said thermistor by direct conduction thereto to supplement the heat produced by said element, said second heat flow path being mounted on said oscillatory circuit.

13. An oscillator circuit comprising, in combination, an electron discharging device having an input and output electrode, an oscillatory tank circuit connected between said input and output electrodes and including an inductance, a resistance device characterized by a negative `temperature coefficient of resistance mounted adjacent said inductance so that said resistance device is responsive to heat emitted from said inductance, a current-actuated heat source, a mass of vmaterial having high thermal conductivity mounted upon said resistance device and arranged to vary the temperature of said resistance device according to the heat emitted byksaid heat source, a second source of unidirectional potential, means for connecting said heat source, said resistance device and said mass in series between said second source and a point intermediate the ends of said inductance,

`said resistance device causing thevoltage appliedfrom said source to said output electrode through said resistance device to vary according to the heat emitted from said heat source and from saidV inductance.-

14. An oscillator circuit as claimed in claim 13 and wherein said heat source includes a second resistance device characterized by constant resistance over a wide range of temperatures.

References Cited in the le of this patent v Y e UNITED STATES PATENTS 1,455,767 

